March 21, 2018 1:37am EDTFebruary 3, 2018 12:19pm ESTThese are the groups that have not only the lockdown guys in the eighth and ninth innings, but a full cadre of arms.Dellin Betances, Kenley Jansen, Andrew Miller(Getty Images)

We've seen the importance of lock-down relievers in the postseason, and we've seen the importance of bullpen depth during the dog days of summer. And bullpens have probably never been more important than they are right now, as managers put shorter and shorter leashes on their starting pitchers.

Think about this: In 2017, relievers across baseball combined to throw 38.1 percent of all the innings pitched for the entire regular season. In 2005, that number was just 32.6 percent. Want raw numbers? In 2017, relievers threw 16,469 2/3 innings. In 2005, relievers threw 14,097. That's 2,372 2/3 more innings that relievers were responsible for in 2017. How does that happen? Here's one clue: In 2017, Chris Sale led all MLB starting pitchers with 214 1/3 innings pitched. In 2005, 27 starters threw at least 215 1/3 innings.

You get the point; 2005 wasn't that long ago, but the approach has changed drastically. So, yeah. Bullpens are important. Really important.

Today, we're going to look at the five best bullpens in baseball. These are the groups that have not only the lockdown guys in the eighth and ninth innings, but a full cadre of arms ready to sustain — or, at least ready to attempt to sustain — a full season with this new philosophy on starting pitching.

5. Boston Red Sox

Why they're here: Yes, closer Craig Kimbrel was mortal in 2016, his first year with Boston. He came into that season with a career 1.63 ERA/1.72 FIP but posted a 3.40 ERA and 2.92 FIP for the Sox. He was back to his old self in 2017; Kimbrel allowed only 33 hits in 69 innings, while posting the lowest walk rate of his career (1.8 per nine innings) and striking out 16.4 hitters per nine. Look at it this way: Kimbrel faced 254 batters during the regular season and he struck out 126 of those "hitters," one strikeout short of exactly 50 percent. That's insane.

So, yeah. That's a good place to start. Joe Kelly had been a bit of an enigma his first of couple years in Boston, but he settled nicely into a bullpen role last year as he learned to harness his triple-digit fastball and put up a 2.79 ERA/3.49 FIP. For this group to reach peak bullpen effectiveness, though, the Sox need a couple of guys to bounce back from injury-wasted seasons. Boston traded for Carson Smith after the young right-hander posted a 2.12 FIP with a 11.8 K/9 ratio in 70 appearances for the Mariners in 2015, but Tommy John surgery (and assorted setbacks) essentially wiped out his past two seasons. Last offseason, the Sox made a deal with Milwaukee that landed Tyler Thornburg, a righty who struck out 12.1 per nine and had a 2.15 ERA in 67 games for the Brewers in 2016. So, of course, he missed all of 2017 with shoulder surgery. Both are expected to be ready this spring, and if — and it's a sizable if — they're healthy, they would be huge additions. Right-handed veterans Matt Barnes, Heath Hembree and Brandon Workman are back, as is Robby Scott, a situational lefty who made 57 appearances. Youngster Austin Maddox did enough in his short stint in the majors (0.52 ERA in 17 games) to earn a spot on the postseason roster as a rookie, which gives him an opportunity to earn innings this spring.

4. Colorado Rockies

Why they're here: The Rockies made it pretty clear that fortifying their bullpen was their top priority this offseason. Colorado GM Jeff Bridich handed out nearly identical three-year, $27 million free-agent deals to Bryan Shaw, the right-hander who has been a stalwart in Cleveland's bullpen the past several seasons (career 3.13 relief ERA, at least 70 appearances each of the past five years), and Jake McGee, the lefty who was really good in the Rockies' bullpen last year (2.93 FIP, 9.1 K/9). And to cap off the fortification, the clincher: They gave Wade Davis a deal that will pay him $52 million over the next three seasons. Davis is 32, but he struck out 12.1 per nine innings as the Cubs' closer in 2017 and locked down 32 of his 33 save opportunities.

That's one hell of a trio for late innings, but that's not all the Rockies have in the pen. Lefty Chris Rusin was really good (2.65) as a multiple-inning lefty last year (85 innings in 60 relief appearances), and lefty Mike Dunn is a reliable veteran (he's averaged 68 appearances the past seven years). Adam Ottavino has a bit of closing experience, and though his 5.06 ERA last year wasn't pretty, 46 of his 63 appearances last year were scoreless. And if you take away one awful outing in June (six earned runs in Los Angeles), his ERA drops a full point. Scott Oberg is back after making 66 appearances last year; his FIP (3.50) was much better than his ERA (4.94).

3. Los Angeles Dodgers

Why they're here: The Dodgers have arguably the best closer in baseball handling the ninth inning (and more when they need it) in Kenley Jansen. Seriously, his production last year was ridiculous; he struck out 109, allowed just 44 hits and only seven walks. Seven. All year. Only one relief pitcher with at least 68 innings has ever posted a number better than Jansen's 15.57 K/BB ratio last year (Dennis Eckersley, at 18.25 in 1990). We don't know exactly how the bullpen will fall in line behind Jansen, mostly because the Dodgers currently have eight starting pitchers on their MLB.com depth chart. Three of those guys will need new roles once the season starts, and it's likely they'll wind up in the bullpen. Maybe Ross Stripling, maybe Brock Stewart, maybe Kenta Maeda. Who knows? If stud prospect Walker Buehler doesn't make the cut, he'll likely go back to Triple-A to continue his development as a starter.

As for known bullpen quantities, Josh Fields and Pedro Baez are back after making at least 57 appearances with an ERA under 3.00 last year. Lefty Tony Cingrani fit nicely into the bullpen mix after he arrived in a trade last year, striking out 28 in 19 1/3 innings with L.A. New addition Tom Koehler has been a starter most of his career, but did nice work out of Toronto's bullpen in limited action last September, when he saw his fastball velocity jump with the move to relief work.

2. Cleveland

Why they're here: Mostly because of the dynamic back-end duo of Andrew Miller and Cody Allen, who have been so good for manager Terry Francona the past few seasons. Miller, the lefty, has a 1.47 ERA, 6.13 K/BB and 0.742 WHIP in 83 regular-season appearances for Cleveland (and a 1.48 ERA in 24 1/3 postseason innings). Allen has been the ninth inning guy, and he was outstanding even before Miller arrived on the scene in a 2016 trade. In his four seasons as Cleveland's primary closer, Allen has a 2.62 ERA and is striking out 12.1 per nine innings; he's yet to squander more than four save opportunities in a season.

From there, the reliable right-handed trio of Zach McAllister (50 games, 2.61 ERA in 2017), Nick Goody (56 games, 2.80 ERA) and Dan Otero (52 games, 2.85 ERA) are all back. And lefty Tyler Olsen, who didn't allow a single run in 30 appearances last year? He's back, too. And expect the bullpen to get a bit of help from an overcrowded rotation — seven guys are listed on the depth chart at the moment, and the two who lose that battle will be key pieces of the bullpen (unless the front office uses that depth to make a deal or two).

1. New York Yankees

Why they're here: Giancarlo Stanton and Aaron Judge have gotten all the headlines this offseason, and rightfully so, but the Yankees' bullpen should be just silly good. David Robertson, who has three 30-plus save seasons under his belt and posted a 1.03 ERA in 30 games after he returned to the Yankees in a midseason trade with the White Sox? Yeah, he's third on the bullpen depth chart. Tommy Kahnle, who was part of that same trade, struck out 12.2 per nine and posted a 2.30 FIP after arriving in New York. And, yeah, there were times last year when Aroldis Chapman and Dellin Betances looked a bit vulnerable, but Betances still struck out 15.1 per nine innings and Chapman didn't allow a run in September or through the first couple of rounds of the AL playoffs. They're both in their Age 30 season and will be just fine.

If Chad Green, who had a 1.83 ERA and struck out 13.4 per nine as a reliever last year, doesn't make the rotation this spring (that's very possible, considering how crowded that rotation is right now), he'll be back in a relief role. Same with Adam Warren, who had a 2.35 ERA in 46 relief appearances. And if the Yankees make a move for Yu Darvish or Jake Arrieta (neither is likely), Jordan Montgomery, who had a 3.88 ERA in 29 starts as a rookie, would be the odd man out in the rotation. And new manager Aaron Boone has a plethora of other bullpen options he and his staff will have to sort through this spring.